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United States Patent |
5,783,262
|
Chaiken
,   et al.
|
July 21, 1998
|
Growth of oxide exchange bias layers
Abstract
An oxide (NiO, CoO, NiCoO) antiferromagnetic exchange bias layer produced
by ion beam sputtering of an oxide target in pure argon (Ar) sputtering
gas, with no oxygen gas introduced into the system. Antiferromagnetic
oxide layers are used, for example, in magnetoresistive readback heads to
shift the hysteresis loops of ferromagnetic films away from the zero field
axis. For example, NiO exchange bia layers have been fabricated using ion
beam sputtering of an NiO target using Ar ions, with the substrate
temperature at 200.degree. C., the ion beam voltage at 1000V and the beam
current at 20 mA, with a deposition rate of about 0.2 .ANG./sec. The
resulting NiO film was amorphous.
Inventors:
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Chaiken; Alison (Fremont, CA);
Michel; Richard P. (Bloomington, MN)
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Assignee:
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Regents of the University of California (Oakland, CA)
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Appl. No.:
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762087 |
Filed:
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December 9, 1996 |
Current U.S. Class: |
427/529; 204/192.11; 427/127 |
Intern'l Class: |
C23C 014/08 |
Field of Search: |
427/529,127
204/192.11
|
References Cited
U.S. Patent Documents
4560577 | Dec., 1985 | Mirtich et al. | 427/38.
|
4604181 | Aug., 1986 | Mirtich et al. | 204/298.
|
4664980 | May., 1987 | Sovey et al. | 428/421.
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4828905 | May., 1989 | Wada et al. | 428/213.
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Other References
UCRL-JC-122960, "NiO Exchange Bias Layers Grown By Direct Ion Beam
Sputtering of a Nickel Oxide Target", R.P. Michel et al., Mar. 1, 1996.
|
Primary Examiner: Pianalto; Bernard
Attorney, Agent or Firm: Carnahan; L. E.
Goverment Interests
The United States Government has rights in this invention pursuant to
Contract No. W-7405-ENG-48 between the United States Department of Energy
and the University of California for the operation of Lawrence Livermore
National Laboratory.
Claims
The invention claimed is:
1. In a process for forming an oxide exchange bias layer, the improvement
comprising:
depositing the oxide layer by ion beam sputtering of a metal oxide target
in an atmosphere consisting of an inert sputtering gas,
whereby ion beam sputtering is carried out in the absence of oxygen gas.
2. The improvement of claim 1, wherein ion beam sputtering is carried out
in the presence of a sputtering gas selected from the group consisting of
argon, xenon, neon, and krypton.
3. The improvement of claim 1, wherein the depositing of the oxide layer
was carried out at a rate of about 0.1-0.2 .ANG./sec.
4. The improvement of claim 1, wherein the metal oxide target is composed
of material selected from the groups consisting of NiO, CoO, NiCoO, FeO
and MnO.
5. The improvement of claim 1, wherein the metal oxide target is composed
of NiO and the sputtering gas consists of argon.
6. The improvement of claim 5, wherein the deposited oxide layer is
composed of NiO, and the ion beam sputtering is carried out using an ion
beam voltage of about 500-1000 V and an ion beam current of about 10-40
mA.
7. The improvement of claim 1, wherein the substrate is heated and held at
a temperature ranging from -125.degree. C. to 200.degree. C. during
deposition of an oxide layer composed of NiO.
8. The improvement of claim 1, additionally including:
providing a substrate on which the oxide layer is to be deposited;
provide a metal oxide target composed of NiO;
providing a sputtering gas consists of argon; and
positioning an ion gun at an angle with respect to the metal oxide target;
and directing an ion beam from the ion gun at an angle onto the metal oxide
target causing sputtering off of target material and depositing of the
sputtered material onto the substrate.
9. The improvement of claim 8, additionally including providing control
means for the ion gun, for positioning the target, for positioning the
substrate, and for controlling the deposition rate of sputtered material.
10. The improvement of claim 8, additionally including providing shield
means for the substrate.
11. The improvement of claim 8, additionally including providing a rotating
means for holding different composition targets.
12. A process for forming antiferromagnetic NiO layers for use in
magnetoresistive readback heads, magnetic random access memories, magnetic
field sensors, etc., comprising:
providing a deposition chamber;
providing an NiO target;
providing a substrate;
directing an ion beam onto the NiO target in the presence of argon gas and
absence of oxygen gas causing sputter deposition of the NiO of the target
onto the substrate.
13. The process of claim 12, additionally including heating the substrate.
14. The process of claim 12, additionally including providing a rotable
means containing the NiO target and at least one additional target of a
different composition.
Description
BACKGROUND OF THE INVENTION
The present invention relates to exchange biasing using antiferromagnetic
oxide layers, particularly to oxide exchange bias layers composed, for
example of NiO, and more particularly to the fabrication of amorphous or
crystalline NiO antiferromagnetic exchange bias layers by ion beam
sputtering a NiO target in pure argon sputtering gas, without oxygen gas.
Antiferromagnetic oxide layers, such as NiO, are used, for example, in
magnetoresistive readback heads to shift the hysteresis loops of
ferromagnetic films away from the zero field axis. The shift brings the
most sensitive part of the magnetoresistance loop close to zero field.
Exchange biasing using antiferromagnetic oxide layers, such as NiO and CoO
is used extensively by the magnetic recording industry in the
magnetoresistive readback heads. Also, unidirectional anisotropy can be
produced in a ferromagnetic film by growing it on an antiferromagnetic
buffer layer, resulting in the above-referenced shift of the hysteresis
loop away from the zero field axis. The most widely used fabrication
process for oxide exchange bias layers has involved reactive magnetron
sputtering of pure metal targets in a partial pressure of oxygen gas. This
requires that the partial pressures of both the oxygen gas and the argon
sputtering gas be controlled, making the fabrication process significantly
more complicated. In addition, oxygen gas can have a detrimental effect on
the other metal layers in the device as well as on the deposition chamber
itself. Finally, the presence of oxygen gas makes it difficult to grow,
for example, the NiO exchange biasing layer on top of the NiFe layer.
Thus, a simpler process to produce oxide exchange bias layers is needed.
The present invention fills that need by producing oxide exchange bias
layers (i.e., NiO, CoO, NiCoO) using a simpler fabrication process which
involves ion beam sputtering of an oxide target using argon (Ar) ions,
with no oxygen gas being introduced into the system.
Ion-beam sputter deposition (IBSD) is an increasingly important method of
fabricating magnetic thin films and heterostructures for both basic
research and industrial applications. While basic researchers are
attracted to IBSD because of its simplicity and versatility, manufacturers
are increasingly choosing IBSD because of the flexible process control it
enables.
Ion-beam sputter deposition is a technique in which material that has been
energetically ejected from a target using an ion beam is deposited onto a
substrate. Therefore, it differs from ion milling, where an ion beam is
focused directly onto a substrate in order to form a pattern on it, and
ion cleaning, whereby a defocused ion beam is used to directly remove
contamination from the surface of a substrate. In IBSD a focused ion beam
is pointed at the sputter target, not at the substrate.
One of the principle differences between IBSD and conventional magnetron
sputter deposition is that the targets are outside the sputter gun. This
is also one of the main advantages of IBSD systems since changing targets
is very easy and almost any large piece of material can be used as a
sputter target. IBSD systems can easily be adapted to deposit material
from targets in the form of rods or pellets, just as with pulsed laser
deposition. Just as routinely an IBSD system can switch between conducting
and insulating sputter targets without any reconfiguration of the
electronics. A neutralization filament or plasma neutralizer must be used
to prevent charging of insulating targets, but these features are
conveniently integrated into commercial ion guns.
A possible advantage of IBSD is that deposition can routinely be performed
with chamber base pressures in the 10.sup.-4 torr range rather than the
10.sup.-3 torr range commonly used for magnetron sputtering. With a well
designed differential pumping scheme, even lower chamber pressures should
be possible during IBSD. One cautionary note is that the maximum rates
obtainable with IBSD may be somewhat lower than with magnetron deposition.
With both magnetron and ion-beam sputter process parameters must be chosen
which discourage incorporation of sputter gas into the deposited film.
Otherwise, degraded film properties may result.
Using ion beam sputtering of an NiO target using Ar ions, amorphous NiO
exchange bias layers having a thickness of 175-500 .ANG. have been
produced, using for example, a silicon (Si) substrate heated to
60.degree.-200.degree. C., an ion beam voltage of 500-1000V and a beam
current of 20 mA, with a deposition rate of about 0.1-0.2 .ANG./sec.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a unidirectional
anisotropy in a ferromagnetic film by growing it on an antiferromagnetic
buffer layer.
A further object of the invention is to grow oxide exchange bias layers
(NiO, CoO, NiCoO) by sputtering of a metal oxide target.
A further object of the invention is to produce amorphous or crystalline
NiO antiferromagnetic exchange bias layers by ion beam sputtering an NiO
target in pure argon sputtering gas.
Another object of the invention is to provide a process for producing NiO
exchange bias layers wherein no oxygen gas is introduced into the system.
Another object of the invention is to provide a process for fabricating NiO
exchange bias layers using ion beam sputtering of an NiO target using pure
argon ions with or without a heated substrate.
Other objects and advantages of the present invention will become apparent
from the following description and accompanying drawings. Basically, the
invention involves the growth of oxide exchange bias layers by ion beam
sputtering of a metal oxide target. In accordance with the invention,
amorphous or crystalline NiO antiferromagnetic exchange bias layers have
been fabricated by ion beam sputtering an NiO target in pure argon
sputtering gas. The process of this invention does not require the
addition of oxygen gas to the sputtering gas, greatly simplifying the
fabrication process. Thus, fabricating exchange bias films of NiO, CoO,
and NiCoO and oxides can be carried out using the process of this
invention. This process can be incorporated into the fabrication of
non-volatile magnetic random access memory, and magnetic field sensors, as
well as for producing antiferromagnetic oxide layers for use in
magnetoresistive readback heads to shift the hysteresis loops of
ferromagnetic films away from the zero field axis, which shift brings the
most sensitive part of the magnetoresistance loop close to zero field.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated into and form a part of
the disclosure, illustrate an embodiment produced by the invention and an
embodiment of an apparatus for carrying out the process of the invention
and, together with the description, serve to explain the principles of the
invention.
FIG. 1 is a partial, cross-sectional view of an embodiment of a device
including an NiO exchange layer formed in accordance with the present
invention.
FIG. 2 is a schematic illustration of an apparatus for carrying out the
invention.
FIGS. 3A and 3B show respective magnetoresistance and magnetization data
for a film grown by ion-beam sputter deposition.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to the formation of oxide exchange bias
layers by ion beam sputtering of a metal oxide target. Exchange biasing of
ferromagnetic films using antiferromagnetic buffer layers is used, for
example, in magnetoresistive sensors to linearize the measured signal and
reduce noise. The coupling to the antiferromagnetic layer produces a
unidirectional anisotropy in the ferromagnetic layer which shifts its
magnetization loop away from the zero field axis. NiO, CoO, and NiCoO are
attractive materials to use for biasing layers due to their high thermal
stability and electrical resistance. NiO films have conventionally been
grown by reactive magnetron sputtering of Ni targets in a partial pressure
of argon and oxygen. The presence of oxygen in the formation process
results in several problems: 1) requiring the control of the partial
pressures of both the argon sputtering gas and the oxygen gas; 2) produces
a detrimental effect on the other metal layers in the device, as well as
on the deposition chamber itself, and 3) makes it difficult to grow the
NiO exchange biasing layer on top of the NiFe layer.
In accordance with the process of the present invention NiO exchange bias
layers have been fabricated using ion beam sputtering of an NiO target
using argon (Ar) ions. No oxygen gas is introduced into the deposition
system. In tests conducted to verify the process, embodiments were
fabricated and which consists of a substrate 10, composed of silicon,
glass, or ceramic, an optional amorphous Al.sub.2 O.sub.3 film 11 having a
thickness of 1000 .ANG., an NiO exchange film or layer 12 having a
thickness of 400 .ANG. or greater, and an NiFe layer or film 13 having a
thickness of 50-300 .ANG.. During fabrication, of the FIG. 1 embodiment,
the temperature of substrate 10 was held between -125.degree. C. to
300.degree. C., the ion beam voltage was 500-1000V and the beam current
was 10-40 mA, and produced a deposition rate of about 0.2 .ANG. on the
substrate 10, with argon gas being at a pressure of 1.times.10.sup.-4 to
4.times.10.sup.-4 Torr. The resulting NiO film 12 was either amorphous or
crystalline. The NiFe film 12 deposited on top of the NiO film 11 showed
the desirable exchange-biased characteristic. A greater deposition rate up
to several .ANG./sec. can be accomplished by using a larger ion gun with a
higher ion current than 40 mA.
The resulting exchange bias field, of the FIG. 1 embodiment, in the easy
axis direction was 180 e at room temperature. While substrate heating is
not necessary, the temperature of the substrate may vary from about room
temperature to 200.degree. C. While NiO processing was carried out to
verify the process of the invention, exchange bias films of CoO, NiCoO,
and other oxides such as FeO, MnO, and mixtures thereof can be fabricated
by this process, using different substrate temperatures and ion beam
parameters. Also, other inert sputtering gases, such as Xe, Ne and Kr can
be used.
FIG. 2 is a schematic illustration of a vacuum system used for carrying out
the ion-beam sputter deposition (IBSD) technique in accordance with the
process of the present invention, and which has been utilized for making
other compositions of magnetic thin films with ion-beam sputter
deposition. See R. P. Michel et al., "NiO Exchange Bias Layers Grown By
Direct Ion Beam Sputtering of a Nickel Oxide Target", UCRL-JC-122960,
bearing a date of Mar. 1, 1996.
In this sputter-up, facing-target apparatus, the ion gun sits at a fixed
angle to targets which are mounted on a stepper-motor controlled carousel.
No attempt was made to control the temperature of the targets. While
insulating material targets may crack under the thermal stress of
sputtering, this causes no problems as long as the target holder keeps the
pieces in place.
In the apparatus shown in FIG. 2 substrates are mounted on a tray which is
controlled using a linear/rotary motion feedthrough. One substrate at a
time is rotated over a hole in a cryoshroud which is centered directly
over the center of the target to be sputtered. Presputtering is performed
while all substrates are rotated away from the deposition position. In
this way the need for a mechanical shuttering system is avoided.
As shown in FIG. 2 the apparatus comprises a deposition chamber or housing
20 containing in a lower section thereof a carousel 21 mounted on a
stepper-motor 22 located external of housing 20 and including target
holders 22 and 23, only two shown containing targets 22' and 23' composed
of NiO and NiFe, for example, with stepper-motor 22 being connected to a
personal computer 24. An ion gun 25 is mounted in housing 20 and connected
to an ion-beam power supply 26, which is connected to controller 24; with
ion gun 25 also connected to an argon gas source 27 via a flow controller
28. A quartz-crystal oscillator (QCO) 28 is mounted in a wall of housing
20 and connected to controller 24 via a deposition controller 29. A
rotable tray 30 having open pockets or holders 31 and 32 for retaining
substrates 33 and 34 is mounted in an upper section of housing 20 and is
controlled using a linear/rotary motion feedthrough indicated by arrow 35,
which may be connected to controller 24. A cryoshield or cryoshroud 36
having an opening 37 is connected to tray 30 and located adjacent OCO 28.
A substrate heater 38 is positioned in housing 20 and above the rotable
tray 30, for heating either substrate 33 or 34. Depending on the materials
involved, other inert sputter gases may be used, such as Xe, Ne and Kr.
In operation, with opening 37 of cryoshield 36, aligned with substrate 33
in holder 31 and centered directly over the center of target 22' using the
sputter-up, facing-target arrangement shown in FIG. 2, the ion gun 25 sits
at a fixed angle with respect to target 22' as shown. Energy from ion gun
25, as indicated by arrows 39 is directed onto target 22' causing sputter
deposition of the material of target 22' onto substrate 33, with the
cryoshield 36 preventing deposition of the target material on the rotating
tray 30. One substrate (33 or 34) at a time is rotated over the opening 37
in the cryoshield 36, and carousel 21 is rotated as desired whereby either
material from target 22' or target 23' is deposited on either substrate 33
or 34. Presputtering is performed while all substrated are rotated away
from the deposition position, or cryoshield 36 is rotated where opening 37
does not align with a substrate holder. While two target holders and two
substrate holders are shown, additional holders may be provided to
selectively deposit material on a greater number of substrates, and/or to
selectively use a greater number of different target materials.
Making a complex structure like a magnetic multilayer requires switching
from one material to another rapidly. With an IBSD system the change of
target is particularly simple. The reason is that there are two processes
going on with an ion gun: the ionization of sputter gas atoms, and the
acceleration of the ions. Both processes are needed to produce the
energetic (.about.keV) ions necessary to eject material from the target.
The acceleration of the ions can be halted within a few seconds by
lowering the voltage on the grids that make up the ion-focusing optics.
Thus an IBSD system can have an "electrical shutter" instead of
electromechanical or pneumatic ones. While the ion-acceleration voltage is
low, the user simply moves a new target into the position in front of the
ion gun. In the deposition chamber shown in FIG. 2, the change of targets
is accomplished using a stepper-motor-driven rotary-motion feedthrough.
FIGS. 3A and 3B show magnetoresistance and magnetization data for a
spin-valve film, one type of film that may be used in a magnetoresistive
readback head. The layering for this film is NiFe50 .ANG./Co10 .ANG./Cu30
.ANG./Co50 .ANG./NiO 500 .ANG./Si substrate. This film was grown via IBSD
using deposition parameters which were separately optimized for the
different constituent materials. One advantage of IBSD is that within a
limited range the ion-beam energy, ion-beam current and background gas
pressure can be varied independently of one another. This allows the film
grower with an automated deposition system to specify different process
parameters for each of the materials to be incorporated into a device.
Experience with IBSD has shown that some technologically important
materials like NiFe have materials properties which are sensitive to each
of the ion-beam parameters as well as to substrate temperature and dc
substrate bias.
The magnetization curve of FIG. 3A shows that the NiFe and Co layers of
spin-valve switch at different characteristic fields. This differential
switching is at the heart of the operation of spin-valve sensors, which
are expected to play a major role in the next generation of magnetic
recording heads and other thin-film sensors. FIG. 3B illustrates the
"giant magnetoresistance" that accompanies the differential switching of
the magnetic layers. Considerably larger magnetoresistance can be achieved
in IBSD films by varying the thickness and number of layers that make up
the spin-valve. IBSD appears to be a viable method for rapid and reliable
fabrication of spin-valve films that incorporate oxide exchange-bias
layers.
It has thus been shown that the present invention enables the formation of
oxide exchange bias layers by ion beam sputtering of a metal oxide target
in an argon sputtering gas, and without introduction of oxygen gas into
the deposition system. This process enables the making of exchange bias
films of NiO, CoO, NiCoO, and other oxides. By elimination of oxygen gas
during the deposition process, the process is made simpler and the
problems associated with oxygen incorporation during deposition ofmetal
films are eliminated. The process of this invention provides
antiferromagnetic oxide layers, which in addition to use in
magnetoresistive readback heads, they can be incorporated in nonvolatile
magnetic random access memory systems and magnetic field sensors.
While a particular embodiment, apparatus and operational sequence, along
with specific materials, parameters, etc. have been set forth to exemplify
and teach the principles of the invention, such are not intended to be
limiting. Modifications and changes may become apparent to those skilled
in the art and it is intended that the invention be limited only by the
scope of the appended claims.
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